Filariasis is an important group of diseases affecting humans for which there is no adequate chemotherapy. From the very limited number of biochemical investigations performed on filarial worms, differences between the various species are known to occur. It is proposed: (1) to inaugurate biochemical investigations of the energy metabolisms of both adult and microfilarial forms of Litomosoides carinii, Dipetalonema viteae (equals witei), Brugia pahangi and possibly Dirofilaria immitis, in an effort to determine which of these is most similar metabolically to the filariids which infect humans; and (2) to study the mode of action of diethylcarbamazine on microfilariae and organic antimonials and arsenicals on the adult stages. Standard isotope and enzyme techniques will be employed to elucidate and compare metabolic pathways. A similar approach in addition to EM autoradiography, and ultrastructural observations are proposed for the study of the effects of antifilarial agents.